Dual service compressor system for conditioning hydrocarbon gas

ABSTRACT

A system for compressing gas from a wellbore that uses a single reciprocating compressor unit to boost pressure of the gas to an intermediate stage, and from the intermediate stage to a final stage. The final stage is at a destination pressure for distribution. Between the intermediate and final stages the gas is treated to remove water and higher molecular weight hydrocarbons so that the gas pressurized to the final stage is compressed natural gas. The reciprocating compressor is made up of a series of throw assemblies that are all driven by a single shaft. Each throw assembly includes a cylinder with a piston that reciprocates within the cylinder to compress and pressurize the fluid therein. The reciprocating compressor can be a non-lube design thereby eliminating lube oil contamination of downstream compressed natural gas or higher molecular weight hydrocarbons.

BACKGROUND OF THE INVENTION

Field of the Invention

The present disclosure relates in general to a system and method forcompressing gas from a hydrocarbon producing well, where the gas iscompressed to an intermediate pressure and to a final discharge pressurewithin a single unit.

Description of Prior Art

Systems for forming compressed natural gas (CNG) typically include abooster compressor that compresses the feed gas to an intermediate stagepressure. While at the intermediate stage pressure, the gas is treatedto remove natural gas liquids, which typically include constituentshaving two or more carbon atoms. The remaining gas, the majority ofwhich generally is made up of methane, is then compressed with a secondcompressor commonly referred to as a CNG compressor. The boostercompressor and CNG compressor can often each have a weight in excess of75,000 pounds and occupy a significant amount of space. CNG compressorsuse electric motors; when disposed in remote locations the motorsrequire onsite generators for their power.

SUMMARY OF THE INVENTION

Disclosed herein is an example of a method of producing natural gas thatincludes providing a reciprocating compressor having a booster cylinderand a compressed natural gas (CNG) cylinder, directing an amount of gasfrom a wellbore to the compressor, compressing the amount of gas in thebooster cylinder to an intermediate stage pressure to define an amountof intermediate stage gas, directing the intermediate stage gas to theCNG cylinder, and compressing the intermediate stage gas in the CNGcylinder to a destination pressure to form compressed natural gas. Themethod may further include treating the intermediate stage gas prior todirecting the intermediate stage gas to the second one of the cylinders.In this example, treating the intermediate stage gas involves separatinghigher molecular weight hydrocarbons from the intermediate stage gas.Further in this example, treating the intermediate stage gas removesmoisture from the intermediate stage gas. Removing moisture from theintermediate stage gas can take place by adding a hygroscopic agent tothe intermediate stage gas. In an embodiment, the booster cylinder ismade up of a first booster cylinder and a second booster cylinder, andwherein a discharge of the first booster cylinder connects to a suctionin the second booster cylinder. In an example, the CNG cylinder is afirst CNG cylinder and a second CNG cylinder, and wherein a discharge ofthe first CNG cylinder connects to a suction in the second CNG cylinder.The reciprocating compressor may include a body, a shaft extendingaxially through the body, pistons in the booster and CNG cylinderscoupled to the shaft, and a motor/engine connected to the shaft, themethod further including activating the motor/engine to rotate the shaftand to reciprocate the pistons in the cylinders. The reciprocatingcompressor may further have a control panel on the body, the methodfurther involving manipulating the control panel to operate themotor/engine. Moisture may be removed from the gas from the wellborebefore directing the gas from the wellbore to the compressor.

Another method of producing compressed natural gas disclosed hereinincludes providing a reciprocating compressor having a body, a shaft inthe body, a series of cylinders that extend radially outward from thebody, and pistons in the cylinders, supplying fluid from a wellbore to aone of the cylinders that is designated as a booster cylinder, creatingintermediate stage fluid by pressurizing the fluid in the boostercylinder, removing moisture from the intermediate stage fluid to formintermediate stage gas, and forming an amount of compressed natural gasby pressurizing the intermediate stage gas in another one of thecylinders. Higher molecular weight hydrocarbons can be removed from theintermediate stage fluid. The series of cylinders can be a multiplicityof booster cylinders. Optionally, the another one of the cylinders is acompression cylinder, and wherein the series of cylinders are amultiplicity of compression cylinders.

Also disclosed herein is a compression system for generating compressednatural gas that has a body, cylinders mounted on the body and pistonsin the cylinders that comprise a booster compressor and a compressednatural gas compressor, a feed line containing fluid from a wellbore andhaving an end connected to a suction side of the booster compressor, asuction side on the compressed natural gas compressor that is in fluidcommunication with a discharge side on the booster compressor via anintermediate circuit, and a discharge line containing compressed naturalgas and connected to a discharge side of the compressed natural gascompressor. The compression system may also have a dehumidificationsystem disposed in the intermediate circuit. Optionally, a tank can bedisposed in the intermediate circuit for removing higher molecularweight hydrocarbons. A crankshaft may be included with the compressionsystem that is coupled with each of the pistons, and a motor/engine canbe included that is coupled with the crankshaft. Further included withthis example is a control system mounted on the body and in signalcommunication with the motor/engine.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the features and benefits of the present invention having beenstated, others will become apparent as the description proceeds whentaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of an example of a system for processingfluid from a wellbore.

FIG. 2 is a schematic example of a dual service compressor for use withthe system of FIG. 1.

While the invention will be described in connection with theembodiments, it will be understood that it is not intended to limit theinvention to that embodiment. On the contrary, it is intended to coverall alternatives, modifications, and equivalents, as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

The method and system of the present disclosure will now be describedmore fully hereinafter with reference to the accompanying drawings inwhich embodiments are shown. The method and system of the presentdisclosure may be in many different forms and should not be construed aslimited to the illustrated embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey its scope to those skilled in the art.Like numbers refer to like elements throughout. In an embodiment, usageof the term “about” includes +/−5% of the cited magnitude. In anembodiment, usage of the term “substantially” includes +/−5% of thecited magnitude.

It is to be further understood that the scope of the present disclosureis not limited to the exact details of construction, operation, exactmaterials, or embodiments shown and described, as modifications andequivalents will be apparent to one skilled in the art. In the drawingsand specification, there have been disclosed illustrative embodimentsand, although specific terms are employed, they are used in a genericand descriptive sense only and not for the purpose of limitation.

An example of a compressed natural gas (CNG) system 10 is schematicallyillustrated in FIG. 1. The CNG system 10 is downstream of a wellheadassembly 12 shown mounted over a wellbore 14 that intersects a formation16. Hydrocarbons, both liquid and gas, from the wellbore 14 are producedthrough the wellhead assembly 12 and transmitted from wellhead assembly12 via a connected production line 18. Production line 18 terminates ina header 20. The header 20 may optionally be the destination for otherproduction lines 22, 24, 26 that also transmit production fluid fromother wellhead assemblies (not shown). A feed line 28 provides acommunication means between the header 20 and CNG system 10. The end offeed line 28 distal from header 20 terminates in a knockout drum 30 andwhich optionally provides a way of separating water and other liquidsfrom the feedline 28. A drain line 32 connects to a bottom of knockoutdrum 30 and directs liquids separated out from the fluid flow in feedline 28. The gas portion of the fluid in feed line 28 directed intoknockout drum 30 exits knockout drum 30 through an overhead line 34shown extending from an upper end of knockout drum 30. The end ofoverhead line 34 distal from knockout drum 30 connects to a suction lineof a compressor 36. In the example of FIG. 1, compressor 36 includes abooster compressor 38 and a CNG compressor 40. In this example, overheadline 34 terminates at a suction end of booster compressor 38 so that thegas in line 34 can be pressurized to an interstage pressure.

The interstage gas discharged from booster compressor 38 is treated inan interstage conditioning system 42. More specifically, a dischargeline 46 provides communication between a discharge side of boostercompressor 38 to a dehydration unit 48. In one alternative, an injectionline 50 for injecting hygroscopic agent into the intermediate stage gasflow stream is shown connected to dehydration unit 48. In one examplethe hygroscopic agent includes triethylene glycol (TEG), and extractsmoisture contained within the interstage gas. A discharge line 52 isshown connected to dehydration unit 48, and provides a means formoisture removal from the intermediate stage gas. Overhead line 54 isshown connected to an upper end of unit 48 and which is directed to aheat exchanger 56. Within heat exchanger 56, fluid from within overheadline is in thermal communication with fluid flowing through a bottomsline 58; where bottoms line 58 connects to a lower end of natural gasliquid (NGL) tank 60. Downstream of heat exchanger 56, overhead line 54connects to a heat exchanger 62. Flowing through another side of heatexchanger 62 is fluid from an overhead line 64, where as shown overheadline 64 attaches to an upper end of NGL tank 60. An optional chiller 66is shown downstream of heat exchanger 62 in line with overhead line 54.Further in the example of: FIG. 1 is a control valve 68 illustrated inoverhead line 54 and just upstream of where line 54 intersects with NGLtank 60. Liquid within line 58 is transmitted to offsite 70, and iscontrolled to offsite 70 via a valve 72 also shown set within line 58.Valve 72 can be manually or motor operated.

Overhead line 64 is shown connected to a suction end of CNG compressor40 and where the gas within overhead line 64 is compressed to a CNGpressure. A discharge line 74 connects to a discharge side of CNGcompressor 40 and provides a conveyance means for directing thecompressed natural gas from CNG compressor 40 to a tube trailer 76.Optionally, a valve 78 is provided in discharge line 74 and forregulating flow through discharge line 74; and to selectively fill tubetrailer 76. Alternatively, each booster compressor 38 may include afirst stage 80 and second stage 82. In this example, discharge fromfirst stage 80 flows through suction of second stage 82 for additionalpressurization. Similarly, CNG compressor 40 contains a first stage 84and second stage 86, wherein gas within first stage 84 is transmitted toa suction side of second stage 86 for additional compression. Examplesexist wherein the booster compressor 38 and CNG compressor 40 arereciprocating compressors and wherein each have a number of throws,wherein some of these throws may be what is commonly referred to astandem throws.

In one example of operation, a multiphase fluid from well 14 flowsthrough lines 18, 20, 28 and is directed to knockout drum 30.Embodiments exist where the fluid flowing through these lines containsat least an amount of flare gas, which might commonly be sent to a flareand combusted onsite. An advantage of the present disclosure is theability to economically and efficiently produce an amount of compressednatural gas that may be captured and ultimately marketed for sale.Liquid within the fluid in line 28 out flows to a bottom portion ofknockout drum 30 and is separated from gas within the fluid. From withindrum 30, the gas is directed into overhead line 34. Line 34 delivers thegas to the suction of booster compressor 38, where in one example thegas is pressurized from an expected pressure between 50 to 100 psig to apressure of 400 psig, and which forms the interstage gas. Gas, which mayinclude hydrocarbons, is directed through line 46 into drum 48. For thepurposes of discussion herein, lower molecular weight hydrocarbons arereferred to those having up to two carbon atoms, wherein highermolecular weight hydrocarbons include those having three or more carbonatoms. To remove moisture from within the interstage gas in line 46,hygroscopic agent is directed from injection line 50 into dehydrationunit 48 and allowed to contact the gas within dehydration unit 48.Alternatively, a molecular sieve 88 may be provided within dehydrationunit 48. Hygroscopic agent, or sieve 88, can then absorb moisture withinthe interstage gas. Sieve 88 may be regenerated after a period of timeto remove the moisture captured within spatial interstices in the sieve88. Regeneration can be by pressure swing adsorption or temperatureswing adsorption.

To remove higher molecular weight hydrocarbons from the interstagegaseous mixture in line 54, the fluid making up the mixture is cooledwithin exchangers 56 and 62 and flashed across valve 68. Cooling thefluid stream, and then lowering the pressure across valve 68, is anexample of a Joule-Thompson method of separation and can condense highermolecular weight hydrocarbons out of solution and into tank 60. Theresulting condensate can be gravity fed from within tank 60 and tooffsite 70. An optional flare 90 is schematically illustrated incommunication with fluid from the wellbore 14 via an end of header 20.Fluid in header 20 can be routed to flare 90 when system 10 is beingmaintained or otherwise out of service.

In alternatives employing the optional chiller 66, the higher molecularweight hydrocarbons are separated from the fluid stream by a mechanicalrefrigeration unit instead of the Joule-Thompson method of gasconditioning. In examples where the Joule-Thompson method is employed,the discharge from the booster compressor 38 can be at about 1,000 psig.In examples using the mechanical refrigeration method, the dischargefrom the booster compressor 38 can be at a pressure of around 400 psig.An advantage of treating the gas at the interstage pressure is theability to remove additional moisture from the gas as well as tooptimize the separation of the higher molecular weight hydrocarbons. Assuch, a higher quality of compressed natural gas can be obtained anddelivered via line 74 into the tube trailer 76. Moreover, a higherquality of NGL can be delivered to offsite 70. In currently knownprocesses, methanol is sometimes added to the gas mixture to prevent theformation of hydrates during the gas treatment process. However, theaddition of methanol is not only costly, but also reduces the qualityand marketability of the end products.

Referring now to FIG. 2 shown is a schematic side sectional example ofthe compressor 36, where the compressor 36 includes a body 90. Throwassemblies 92, 94, 96, 98 are shown coupled to the body 90 and eachalong a path generally transverse to an axis of the body 90. Cylinders100, 102, 104, 106 are shown respectively in each of the throwassemblies 92, 94, 96, 98. Shown in each of the cylinders 100, 102, 104,106 are pistons 108, 110, 112, 114, which reciprocate in the cylinders100, 102, 104, 106 to compress gas within the cylinders 100, 102, 104,106. Piston rods 116, 118, 120, 122 respectively connect pistons 108,110, 112, 114 to a crankshaft 124 shown projecting axially through thebody 90. The crankshaft 124 is driven by a motor 126 shown optionallymounted to the body 90. Operating the motor 126 causes rotation of thecrankshaft 124, which in turn reciprocates pistons 108, 110, 112, 114within their respective cylinders 100, 102, 104, 106. In one example,the motor 126 includes an internal combustion engine that can be poweredby gasoline, gas from the wellbore 14, another combustible material, orcombinations thereof. In another alternative, the motor 126 can beelectrically powered.

Further shown in the example of FIG. 2 is that throw assemblies 92, 94,are included in the booster compressor 38 portion of compressor 36. Inthis example overhead line 34 terminates in throw assembly 92, so thatgas exiting overhead line 34 can be compressed by reciprocation ofpiston 108 within cylinder 100. Gas being compressed in cylinder 100 bypiston 108 is transmitted to throw assembly 94 via line 128. Gas exitingline 128 into cylinder 102 can be compressed by reciprocating piston110. Gas compressed within cylinder 102 exits into discharge line 46,where it is transmitted to interstage conditioning system 42.

Throw assemblies 96, 98 are shown in CNG compressor 40 portion ofcompressor 36. As shown, overhead line 64 terminates at throw assembly96 so that interstage gas from interstage conditioning system 42 istransmitted into cylinder 104. Reciprocation of piston 112 in cylinder104 compresses gas exiting overhead line 64. Gas compressed in thecylinder 104 is transmitted to throw assembly 98 via line 130 shownhaving an upstream end connected to cylinder 104 and a downstream endconnected to cylinder 106. Piston 114 compresses the gas exiting line130 into cylinder 106, which is then discharged into discharge line 74.A control panel 132 for sending controls to the compressor 36, and/ormotor 126 is shown adjacent body 90 and connects to body 90 via bus 134.In and embodiment, bus 134 provides connection for transmitting signalsand/or power to body 90 and motor 126 from control panel 132. Furthershown is a power line 136 connected to motor 126, which can convey fuelto motor 126 in embodiments when motor 126 is an internal combustionengine. Alternatively, power line 136 can provide electricity to motor126 when motor 126 is powered by electricity.

The present invention described herein, therefore, is well adapted tocarry out the objects and attain the ends and advantages mentioned, aswell as others inherent therein. While embodiments of the invention havebeen given for purposes of disclosure, numerous changes exist in thedetails of procedures for accomplishing the desired results. In oneexample the compressor is a non-lube design, an advantage of which isthe reduction of oil and associated equipment requirements, e.g. daytank, strainer, and/or heavy weight oil. A non-lube design can preventoil carry over to downstream equipment like NGL storage tank, tubetrailer, molecular sieves, etc., which eliminates the need of filtrationequipment for critical processes and alleviates any operational issuessuch as contamination, catalyst degradation and the like. Moreover, oilcost savings that results in direct operating expenditures saving forend users. An additional advantage is that a non-lube design eliminatesthe need for forced feed lubrication system (pumps, PSV, internalgearing, labor etc.) to all cylinders, and packing. It also eliminatesthe auxiliary components/instrumentation such as tubing, check valves,poppet valves, distribution blocks, no-flow switch etc. This would inturn reduce the overall compressor price to customer. The non-lubecylinder design can implement non-metallic wear resistant materials forinternal moving components and by the use of appropriate clearances tomaximize heat dissipation in the absence of lube oil. These and othersimilar modifications will readily suggest themselves to those skilledin the art, and are intended to be encompassed within the spirit of thepresent invention disclosed herein and the scope of the appended claims.

What is claimed is:
 1. A method of producing compressed natural gas, themethod comprising: providing a reciprocating compressor having a boostercylinder and a compressed natural gas (CNG) cylinder; directing anamount of gas from a wellbore to the compressor; compressing the amountof gas in the booster cylinder to an intermediate stage pressure todefine an amount of intermediate stage gas; directing the intermediatestage gas to the CNG cylinder; and compressing the intermediate stagegas in the CNG cylinder to a destination pressure to form compressednatural gas.
 2. The method of claim 1, further comprising treating theintermediate stage gas prior to directing the intermediate stage gas tothe second one of the cylinders.
 3. The method of claim 2, whereintreating the intermediate stage gas comprises separating highermolecular weight hydrocarbons from the intermediate stage gas.
 4. Themethod of claim 2, wherein treating the intermediate stage gas comprisesremoving moisture from the intermediate stage gas.
 5. The method ofclaim 4, wherein removing moisture from the intermediate stage gascomprises adding a hygroscopic agent to the intermediate stage gas. 6.The method of claim 1, wherein the booster cylinder comprises a firstbooster cylinder and a second booster cylinder, and wherein a dischargeof the first booster cylinder connects to a suction in the secondbooster cylinder.
 7. The method of claim 1, wherein the CNG cylindercomprises a first CNG cylinder and a second CNG cylinder, and wherein adischarge of the first CNG cylinder connects to a suction in the secondCNG cylinder.
 8. The method of claim 1, wherein the reciprocatingcompressor comprises a body, a shaft extending axially through the body,pistons in the booster and CNG cylinders coupled to the shaft, and amotor connected to the shaft, the method further comprising activatingthe motor to rotate the shaft and to reciprocate the pistons in thecylinders.
 9. The method of claim 8, wherein the reciprocatingcompressor further comprises a control panel on the body, the methodfurther comprising manipulating the control panel to operate the motor.10. The method of claim 1 further comprising removing moisture from thegas from the wellbore before directing the gas from the wellbore to thecompressor.
 11. The method of claim 1, wherein the reciprocatingcompressor is a non-lube design.
 12. The method of claim 11, wherein themotor comprises an internal combustion engine powered by gas from thewellbore.
 13. A method of producing compressed natural gas, the methodcomprising: providing a reciprocating compressor having a body, a shaftin the body, a series of cylinders that extend radially outward from thebody, and pistons in the cylinders; supplying fluid from a wellbore to aone of the cylinders that is designated as a booster cylinder; creatingintermediate stage fluid by pressurizing the fluid in the boostercylinder; removing moisture from the intermediate stage fluid to formintermediate stage gas; and forming an amount of compressed natural gasby pressurizing the intermediate stage gas in another one of thecylinders.
 14. The method of claim 13, further comprising removinghigher molecular weight hydrocarbons from the intermediate stage fluid.15. The method of claim 13, wherein the series of cylinders comprises amultiplicity of booster cylinders.
 16. The method of claim 13, whereinthe another one of the cylinders comprises a compression cylinder, andwherein the series of cylinders comprises a multiplicity of compressioncylinders.
 17. A compression system for generating compressed naturalgas comprising: a body; cylinders mounted on the body and pistons in thecylinders that comprise a booster compressor and a compressed naturalgas compressor; a feed line containing fluid from a wellbore and havingan end connected to a suction side of the booster compressor; a suctionside on the compressed natural gas compressor that is in fluidcommunication with a discharge side on the booster compressor via anintermediate circuit; and a discharge line containing compressed naturalgas and connected to a discharge side of the compressed natural gascompressor.
 18. The compression system of claim 17, further comprising adehumidification system disposed in the intermediate circuit.
 19. Thecompression system of claim 17, further comprising a tank disposed inthe intermediate circuit for removing higher molecular weighthydrocarbons.
 20. The compression system of claim 17, further comprisinga crankshaft coupled with each of the pistons, and a motor coupled withthe crankshaft.
 21. The compression system of claim 20, furthercomprising a control system mounted on the body and in signalcommunication with the motor.